Compositions containing water and lipophilic substances as major components are used for various hard surface applications, e.g. for the treatment of metallic surfaces. In case of metallic surfaces, they are applied as metal working fluids, lubricants, rust preventives, cleaners or permanent coating compositions.
In these compositions, the functionality of water on the one hand side and of the lipophilic substances on the other hand side are combined—often in a synergistic manner. For instance, in case of metal working fluids, water functions as cooling agent, whereas, the lipophilic substances, especially oils, serve for enhancing lubricity.
Since water is not mixable with lipophilic substances per se, surfactants have to be used to enable mixing.
For water/oil/surfactant emulsion systems, there are only four thermodynamically stable phases, the so called Winsor Phases (Winsor, P. A. Trans. Faraday Soc. 1948, 44, 376).
They are easy to switch to one another through the so called phase transition process by various external stimuli like temperature, pressure, pH or contaminants (Bellocq, A. M.; Biais, J.; Bothorel, P.; Clin, B.; Fourche, G.; Lalanne, P.; Lemaire, B.; Lemanceau, B.; Roux, D. Adv. Colloid Interface Sci. 1984, 20, 167; Fletcher, P. D. I.; Howe, A. M.; Robinson, B. H. J. Chem. Soc. Faraday Trans. 1 1987, 83, 985; Olsson, U.; Wennerström, H. Adv. Colloid Interface Sci. 1994, 49, 113; Bourrel, M.; Schechter, R. S. ‘Microemulsions and Related Systems’ Marcel Dekker, 1988, New York; Shinoda, K.; Saito, H. J. Colloid Interface Sci. 1969, 34, 238).
During the phase transition, surfactants must move around to build different structures like micelles or continuous bilayers (Olsson, U.; Wennerström, H. Adv. Colloid Interface Sci. 1994, 49, 113; Israelachvili, J. N. Colloids Surf. A 1994, 91, 1).
However, random movement or migration of the surfactants causes instability of the emulsion and eventually ends up with detrimental phase separation as one of the phase transition behaviors: The faster the transition, the less stable the emulsion system.